The invention is related to a horn for the primary focus illumination of a reflector antenna with a horn flange which is arranged at the output end of a tubular circular waveguide and which widens in a funnel-shaped manner from the throat lying on this waveguide and which has on its inner funnel side grooves oriented in parallel with the axis of the input waveguide.
For such a known horn (as in German published patent application No. DE-OS 3144 319) in which the input waveguide ends precisely with the horn flange throat, by the grooves arranged in parallel to the axis and around the feeding waveguide, a structure is proposed to enable a very precise production of the grooves.
Specially, by such an accurate dimensioning a rather high suppression of the cross polarization is obtained. This known feed horn is specially designed in order to produce a radiation pattern with as low as possible cross polar sidelobes, whereas the aperture illumination or the covering of the reflector antenna to be fed by this horn is less important.
This invention is based on the task of improving the illumination of deep reflector antennas or mirrors, specially for f/D ratios&lt;0.35, where f corresponds to the focal length and D to the aperture diameter of the reflector, in such a manner that under conservation of the least possible cross polarization a high aperture efficiency combined with a high spill over efficiency and a high side lobe suppression is obtained.
This task is by this invention solved by choice of the half opening angle .theta..sub.o of the flange enclosed between the waveguide axis and the inner envelope of the horn grooves to range in the region 70.degree.&lt;.theta..sub.o&lt; 80.degree. and by a forward offset of the TE.sub.11 mode circular waveguide aperture with respect to the horn throat, where this offset of the waveguide is adjusted to obtain the optimal angular width of the horn pattern suitable to the f/D ratio of the reflector.
Ideally, the illumination of covering of the reflector antenna should consist of a constant illumination power across the whole reflector aperture and a step to zero at the reflector rim. This would require a conical-formed field strength (power) characteristics of the feed horn, which has constant field strength inside the opening angle of the reflector. Unfortunately, this ideal case cannot be realized and the uniformity of the illumination especially for deep reflectors is more and more difficult to attain. On the other hand, deep reflectors with f/D&lt;0.35 are of increasing interest, because the horn, being the feed, is more shielded against ground radiation producing additional thermal noise than in shallow reflectors. But it could surprisingly be shown by the invention that appropriate dimensioning of the horn flange opening angle and a suitable "matched" offset of the waveguide against the horn flange throat leads to extraordinary advantageous illuminations e.g. high aperture efficiencies between 50 and 60.degree./o and and a very high spillover efficiency (spillover about 2.degree./o) and a low sidelobe level of about -25 dB (compared against main lobe level). As, further, the cross polarization is considerably suppressed, i.e. the horn radiation characteristic practically shows high cylindrical symmetry, this invented feed is especially useful for circular polarized waves as are e.g. radiated by transmitters of direct TV satellites. The definition of the above mentioned parameters such as: aperture efficiency, spillover efficiency or sidelobe level are in correspondence with common international standards of antenna engineering as e.g. contained in JOHNSON, R. C., JASIK, H. Antenna Engineering Handbook, McGraw Hill, New York 1984, pages 1-5 to 1-7 or RUDGE, A. W., MILNE, K., OLIVER A. D.,KNIGHT,P.: The Handbok of Antenna Design, Peregrinus, London 1982, Vol. 1 pages 21-24.
Particularly good results for deep reflectors shall be obtained in a more narrow angular region of the horn flange opening angle, which is characterized by the domain 73.degree..ltoreq..theta..sub.o .ltoreq.76.degree..
Similarly, for the waveguide aperture offset a favoured domain was found which is characterized by the region -0.25.ltoreq.L/.lambda..sub.o .ltoreq.+0.35 where .lambda..sub.o is the free space wavelength and L is the perpendicular distance between the aperture plane of the horn flange and the aperture plane of the wave guide and the sign of L is positive for distances outside the inner horn flange volume enclosed between the funnel-shaped horn flange and the horn flange aperture plane and negative for distances inside of the horn flange volume.
Particularly, this invention is useful in the context of a feed application using a circular feeding waveguide. In this context it is sensible to choose a conical horn flange which has rotational symmetry to the axis of the waveguide. Especially, a preferred construction consists in a horn flange in the form of a cone of revolution.
A further option of this invention is the extension of the idea of the invention to depart from a single and fixed position of the waveguide offset (L), to the possibility of varying the value of the offset and to adjust it to different positions corresponding to changing requirements. This embodiment is characterized by the fact that the horn flange is adapted to axially slide on the feeding waveguide.
To constructionally realize this idea an embodiment is provided in which the horn flange is arranged on a cylindrical sheath which is fittingly guided on the outer surface of the waveguide. By this embodiment the continuity of the radio frequency connection of the horn flange with the waveguide is guaranteed and a variation of the waveguide aperture offset by shifting of the horn flange is rendered possible. To secure a definite radio frequency connection the horn flange is provided with a contact spring sliding on the outer surface of the waveguide.
In order to realize an accurately controllable shift of the horn flange on the waveguide, further, an electrical drive unit is provided. This arrangement is built in such a manner, that the sheath of the horn flange comprises a rack which engages a pinion which is driven by an electrical motor which is stationary with respect to the waveguide.
Finally, some dimensioning regions normalized to the wavelength of operation and providing particularly useful practical realizations of this invention have been found. These dimensions consist in the outer diameter of the horn flange d.sub.ges, the inner diameter of the waveguide d.sub.TE.sbsb.11, the axial groove depth s, the radial groove distance b and the radial groove thickness t with these regions lying in the ranges
1.86.ltoreq.d .sub.ges /.lambda..sub.o .ltoreq.3.6 PA1 0.59.ltoreq.d.sub.TE.sbsb.11 /.lambda..sub.o .ltoreq.0.82 PA1 0.25.ltoreq.s /.lambda..sub.o .ltoreq.0.35 PA1 0.07.ltoreq.b /.lambda..sub.o .ltoreq.0.12 PA1 0.016.ltoreq.t /.lambda..sub.o .ltoreq.0.024,
where .lambda..sub.o is the operation wavelength in free space.